Conventionally, for the purpose of improving fuel economy of a vehicle, a power supply system having a regenerating function of converting kinetic energy during braking (hereinafter, referred to as “braking energy”) into electric energy by a generator and storing the obtained electric energy, thereby collecting and reusing the braking energy has been proposed (patent document 1).
An example of such a power supply system is shown in FIG. 8. Herein, an example of only a minimum configuration and operation for collecting and reusing braking energy is described.
When a vehicle is braked and generator 1 converts braking energy into electric energy, condenser 2 coupled to generator 1 is charged. The charged electric power is charged to battery 4 via voltage step up and down circuit 3.
Note here that electric energy of generator 1 is once stored in condenser 2 and then charged to battery 4. This is because battery 4 cannot correspond to a rapid charge. Therefore, it is necessary to have a configuration for once storing electric energy in condenser 2 capable of corresponding to rapid charge and discharge, and then charging battery 4 in accordance with the charging rate of battery 4 by voltage step up and down circuit 3.
The electric power charged to battery 4 is supplied to and consumed by load 5.
With such a configuration and operation, it is possible to collect braking energy, which has been wasted as heat during braking. Therefore, the energy efficiency of an entire vehicle can be improved, realizing improvement of fuel economy and reduction of exhaust gas.
A vehicle having such a power supply system uses some load 5 (for example, an engine control circuit essential to move a vehicle) simultaneously mounted on the vehicle even during braking.
In this case, when current consumed by load 5 to be used is small, electric power generated by generator 1 can be charged to condenser 2 and supplied to load 5. However, in the case where a large number of loads 5 are used simultaneously, entire current consumption becomes extremely large. Therefore, all of the electric power generated by generator 1 is directly supplied to load 5.
Therefore, voltage step up and down circuit 3 must be designed so as to allow a maximum current that can be consumed by load 5 to flow.
Thus, when maximum current flows, heat generation of circuit components (in particular, the below-mentioned switching element or inductance element) built in voltage step up and down circuit 3 is increased. Therefore, it is necessary to provide a heat dissipating component such as a large-size heat sink, for efficiently dissipating the generated heat. Consequently, the size of voltage step up and down circuit 3 is increased, resulting in increasing the size of an entire power supply system.
[Patent document 1] Japanese Patent No. 2833087